The effects of 4 and 8 weeks fasting at 16 °C were studied in rainbow trout, Salmo gairdneri Richardson. After 4 and 8 weeks, the wet weights of the intestine of fasted animals are respectively 64% and 69% lower than those of fed animals. These effects especially concern the mucosal tissue. Glycine absorption (0.5 and 10 m m ) was studied using an in vivo perfusion technique. After 4 weeks, the absolute amounts of 0.5 m m glycine absorbed by fasted and fed fish are similar. With 10 m m glycine, the absorption is slightly lower in fasted trout (−19%). After 8 weeks these differences are more marked, with glycine concentrations of 10 m m (−42%). Results expressed per 100 g body weight showed that these differences result partly from a weight gain of fed trout. Absorption expressed in terms of weight of dry intestine is higher in 4 and 8 weeks fasted animals, principally for the lower amino acid concentration (+61% and +111%). Larger differences were apparent when the absorptions were expressed in terms of dry weight of mucosal tissue (+122% and +225%).